In recent years, variable wavelength light sources of which the wavelength is variable in a wide range and of which the line width of the wavelength is as narrow as 100 kHz or less (particularly, a tunable laser source: TLS) have been vigorously developed for digital coherent communication of which the market scale has expanded due to the expansion of the long distance, large capacitance optical transmission systems. As an example of such a variable wavelength light source, a tunable laser can be cited where a silicon platform that is formed of a silicon-based material so as to have a wavelength filtering function and a semiconductor optical amplifier (SOA) that is formed of a compound semiconductor are combined.
FIG. 10 is a schematic diagram illustrating the configuration of a conventional tunable laser source. A semiconductor optical amplifier 71 and a resonator 72 form a tunable laser 70. The resonator 72 is formed of a combination of an optical waveguide made of a silicon photonic wire formed on an SOI substrate, a ring resonator and a loop mirror. In this case, the cleavage surface of the semiconductor optical amplifier 71 on the output end side and the loop mirror form a laser resonator so that the light emitted from the semiconductor optical amplifier 71 enters into the resonator 72 so as to be amplified with a specific wavelength being resonated. Part of the amplified light is outputted from one end of the semiconductor optical amplifier 71 as output light. The ring resonator in the resonator 72 is controlled by a heater or the like that is provided on the ring resonator so that the wavelength of the output light can be tuned.
It is necessary in tunable laser sources to control the output power and the wavelength. For this purpose, a mechanism for monitoring the power and the wavelength of the output light is provided. Typically, part of the output light is split by a first optical splitter (Tap 1) 73 and further split by a second optical splitter (Tap 2) 74 so that two beams that have been split by the second optical splitter 74 are respectively incident into a power monitor 75 and a wavelength locker 76. In the case where the wavelength locker 76 detects a change in the wavelength, the output thereof drives the heater provided on the resonator 72 so as to offset the wavelength fluctuation.